For the purpose of strength improvement and weight reduction, high-strength steel sheets are not infrequently used. When carrying out cold-press, a high-strength steel sheet may considerably spring back as a natural result of its high strength, thereby often causing an issue in shape-fixability. To solve this issue, one may select hot-press instead of cold-press and exploit close contact with dies to carry out quenching and hardening of the sheet so as to increase hardness of the press-formed product. This is referred to as die-quenching, alternatively, press-hardening or hot stamping.
In die-quenching, a steel sheet is heated up to a proper temperature beyond the Ac3 point, 1000 degrees C. for example, so that its structure turns into austenite. Next the steel sheet is taken out of the furnace and, while being air-cooled, is treated with pressing at a proper temperature where the austenite phase is still stable, 800 degrees C. for example. Then quenching resulting from close contact with dies causes the martensite transformation, thereby hardening and strengthening the press-formed product. Its tensile strength is, for example, about 1470 MPa and its Vickers hardness HV is about 440. More specifically, a steel sheet formed by this method has a sufficient strength. The steel sheet under forming is sufficiently soft because the pressing is carried out in a hot process, thereby alleviating the issue of spring-back and producing a precise shape.
When a product formed by die-quenching is to be further machined, or processed in any way, another issue will arise. Japanese Patent Application Laid-open No. 2003-328031 reports that increase of hardness at surfaces of the formed product increases shearing resistance and therefore makes it difficult to carry out piercing or trimming.